[j-nsp] <COS Query> While imposing MPLS labels what value is set on EXP bits

[Adam Vitkovsky]

Hi Amarjeet,

The documentation is a bit unclear on this but it appears that on Juniper MX Trio the default behaviour is that the IP precedence bits are written into the EXP field when an EXP rewrite rules are not in place so to get a predictable behaviour you should be using an explicit MPLS rewrite rules on all core-facing interfaces.
However Juniper M and T series have some predefined default MPLS EXP rewrite rules according to a default BA classifier rules.
You can manually apply a rewrite rule to MPLS and IPv4 packet headers simultaneously.
But by default (and that is a good practice) customer IP precedence markings are untouched while the IP packets are traversing MPLS backbone.
Though one think I'm not sure is if the EXP bits are set on all imposed labels by or just the top label by default.
 

adam
> -----Original Message-----
> From: juniper-nsp [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf
> Of Mark Tinka
> Sent: 02 March 2015 05:46
> To: juniper-nsp at puck.nether.net
> Subject: Re: [j-nsp] <COS Query> While imposing MPLS labels what value is
> set on EXP bits
> 
> 
> 
> On 1/Mar/15 19:14, Amarjeet Singh wrote:
> > Folks - I've query on COS in MPLS VPN environment.
> >
> > While receiving Packets ingress from CE, let's say TOS field in IP header
> > is set to IPP-4.
> > And i've a MF that is classifying IPP-4 into FC-4 + PLP-LOW.
> > + There is MPLS Rewrite rule on egress interface that is matching
> > FC-4+PLP-low and setting EXP bits to 4 (100)  while relaying packet to next
> > hop.
> >
> > EXP rewrite happens only on MPLS encapsulated packets before that
> packet
> > should be MPLS encapsulated first.
> >
> >  While receiving IP packets coming from CE and imposing 2 Label stack on it
> > (Transport/VPN label) what value is set on EXP bits of Labels? Does the TOS
> > value in IP header of IP packet is copied to MPLS EXP bits?
> >
> > How does the MPLS classification happens to set FC+PLP on MPLS packets
> > before hitting MPLS EXP rewrite rule on egress?
> 
> Most vendors, by default, will copy the first 3 bits of the DSCP value
> into the EXP field, if there is no explicit remarking of the EXP field
> on ingress.
> 
> Since IPP is 3 bits only, the IPP bits get copied to the EXP field verbatim.
> 
> I'm not sure how this works if you're remarking on egress (classic
> Junos). We remark on ingress, separately for DSCP and EXP.
> 
> Mark.
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